(1) Field of the Invention
The present invention is directed to improved membrane switch assemblies. Specifically, this invention relates to a membrane switch assembly having internal cavities which are vented to the ambient atmosphere through a porous or air permeable structure.
(2) Description of the Prior Art
Prior art membrane switch assemblies of the type employed in miniaturized keyboards have customarily been constructed by laminating an apertured spacer sheet between two substrates which support printed circuits. The substrates, at least one of which will be flexible, are positioned so that circuit patterns thereon face each other. The switches are defined by locating the spacer sheet apertures so that, with the application of pressure to one of the substrates, appropriate portions of the printed circuits can be made to contact each other. These prior art membrane switch assemblies were usually constructed so that the switch cavities or chambers formed by the apertures within the spacer sheet were permanently sealed from the surrounding environment. These cavities were filled with a gas, typically air.
The above-discussed prior art method of constructing membrane switch assemblies has certain disadvantages. A major disadvantage which results from hermetically sealing the cavities defined by the spacer sheet apertures occurs when there is a change in the external fluid pressure, the atmospheric pressure for example. If a machine which incorporates the membrane switch assembly is located at an altitude where the outside atmospheric pressure is less than the pressure within the sealed cavities, the greater internal pressure exerts an outward force upon the layers of the switch laminate. The result of this outward expansion is that there is a cushioning effect to the operation of the individual keys. With a sufficiently large pressure differential, it becomes difficult for the operator to determine whether the key has been activated. In the extreme situation, when the difference between the outside atmospheric pressure and the pressure within the cavities is quite large, the membrane switch assembly may become distorted with structural damage possibly being caused by the increasing pressure on the laminate walls caused by the outward expansion.
A similar result occurs when the outside atmospheric pressure becomes greater than the pressure within the cavities. This will occur, for example, when the mechanism incorporating the membrane switch assembly is operated in an environment where the ambient pressure is greater than that where the laminate was constructed. The result would be that the force exerted upon the wall of the laminate by the outside atmospheric pressure would move the walls of the laminate inwardly. The usual effect of this pressure differential would not be as significant as when the atmospheric pressure is less than the internal pressure. However, in the extreme condition when the pressure differential between the outside atmospheric pressure and the internal pressure becomes great, the switch might be activated.
It is to be observed that, even under normal operating conditions, the gas which is in the cavities resists compression of the walls of the laminate when a user tries to activate the keys. This results in a cushioning effect which is felt by the user of prior art membrane switch assemblies. While under certain circumstances a cushioning effect may be desirable, it may also reduce the users ability to activate a switch, by depressing a key for example, or detect whether a switch has been actuated.
Several methods have been proposed and/or utilized to try to alleviate the above-discussed disadvantges of prior art membrane switch assemblies. One proposed prior art method involves incorporating internal channels within the laminate between the cavities. This allows displacement of the fluid medium between the internal cavities of the membrane switch assembly. When one switch is activated the fluid within the spacer sheet defined cavity associated with that switch is displaced by the downward force of the membrane wall and will flow through the channels into one or more other cavities. While this will help to minimize the cushioning effect caused by the resistance of the internal pressure to the downward depression of the membrane wall, it will not alleviate the problems associated with an internal/external pressure differential.
It has also been proposed to equalize the internal pressure with the external pressure by establishing fluid communication between the ambient atmosphere and the interior of the switch assembly by providing a hole in the outer layers of the membrane switch assembly; commonly referred to as a through-hole. This through-hole in the laminate of the switch assembly allows air to flow freely into and out of the assembly's cavities. While this technique would solve the problems associated with the pressure differential between the external/internal pressures, it creates some of its own disadvantages. The major of these disadvantages becomes apparent with the incorporation of the completed membrane switch assembly into a final product. The through-hole vents would typically be provided through the entire switch assembly. Although holes in the front surface of the assembly may be sealed off, for example by indicia bearing sheets, the holes at the back surface must remain clear. This causes difficulties when installing the switch laminate into products such as calculators, microwave ovens, thermostatic controls, etc. The membrane switch assembly would, to keep the through-hole open, have to be either spatially separated from the surrounding housing or the surrounding housing would have to be provided with corresponding holes to allow for a free flow of air into and out of the through-holes. This requires additional manufacturing steps or a larger housing to provide the spatial separation. Furthermore, since many membrane switch assemblies are secured within the final product through uses of adhesives, during manufacturing, special care would be required to avoid having the adhesive flow into or seal off the through-hole vents. Finally, free flow between the ambient atmosphere and the interior of the switch assembly enhances the possibility of dirt or other contaminants reaching the switch contacts and causing faulty operation.